(1) Field of the Invention
The present invention relates to a liquid crystal display and in particular to a liquid crystal driving system and a method for driving a liquid crystal display.
(2) Description of the Prior Art
The main advantages of liquid crystal displays are easy to achieve high resolutions and slim sizes. Therefore, liquid crystal displays are widely used in notebook computers. And because of constant developments in large-sized displays, liquid crystal displays also become the main-stream monitors for desktop computers. Moreover, liquid crystal display televisions are popular commodities in the television market.
FIG. 1 is a basic diagram of a liquid crystal display 10. A liquid crystal panel 12 comprises a plurality of pixel units 121. By providing a specific electrical voltage to the liquid crystal molecules of each pixel unit 121, the spinning angle of the liquid crystal molecules can be changed. With a backlight 14 underneath the liquid crystal panel 12, each pixel unit 121 in the liquid crystal panel 12 has a different penetration ratio to light. The plurality of pixel units 121 forms an array providing visual frames for a user seeing. In more details, each pixel unit 121 comprises three sub-pixels 121R, 121G and 121B. Each sub-pixel (121R, 121G or 121B) handles one color element (red or green or blue) within a pixel unit 121.
One of the technical bottlenecks in liquid crystal display technology is related to the physical property of liquid crystal molecules. When the particular electrical voltage as described above is applied to liquid crystal molecules, the liquid crystal molecules require a period of time for reacting and turning its initial angle to a different angle (with the resulting angle related to the particular electrical voltage input applied). When displaying a moving image, the response time of liquid crystal molecules can not catch up the screen refresh rate, thus resulting in delay and poor display quality. Therefore, shortening the response time of liquid crystal molecules is an important issue in liquid crystal display technology.
Liquid Crystal Display overdrive (LCD overdrive) is a method to shorten the display lag time. The method provides a higher (or lower) electrical voltage to liquid crystal molecules and forces the liquid crystal molecules to turn to the required angle within a prearranged period of time. The following FIG. 2 explains the liquid crystal display overdrive technology. In FIG. 2, the vertical axis represents the spinning angle of liquid crystal molecules, and the horizontal axis represents time. When applying a control voltage VCL to a pixel unit, the liquid crystal molecules require a period of time t2 for changing from the angle θ0 to the angle θ1 corresponding to the control voltage VCL. The liquid crystal display overdrive technology is to apply an overdrive control voltage VOD (VOD is the control voltage of angle θ2) to a pixel unit in advance, and thus the time shortens to t1 for turning the liquid crystal molecules to the angle θ1, thus resulting in reducing the response time of liquid crystal molecules.
Referring to FIG. 3, FIG. 3 shows a traditional liquid crystal driving system with an overdrive function. A liquid crystal driving system 20 receives a video signal 21. The video signal 21 could be from either DVD or VCD players, computer video outputs, or other signal sources. The video signal 21 is a gray scale signal, and typically able to display 256 different gray scales within one sub-pixel. When processing various kinds of input signals, integers (E.g. 0, 1, 2, 3 . . . 254, and 255) are usually used to represent each gray scales. However, in implementation, binary numbers are used instead of integer numbers.
The liquid crystal driving system 20 can finally produce a control voltage VCL and an overdrive voltage VOD to drive the liquid crystal display panel (FIG. 1 mark 12). The control voltage VCL is generated according to a driving signal 21′. The overdrive voltage VOD is generated according to an overdrive signal 23. The liquid crystal driving system 20 comprises an overdrive unit 22, generating the driving signal 21′ according to the video signal 21 and generating the overdrive signal 23 according to the video signal 21 and an overdrive signal look-up table (FIG. 4). The overdrive signal look-up table is built within the overdrive unit 22. The Driving signal 21′ and overdrive signal 23 are digital gray scale signals. The liquid crystal driving system 20 further comprises a selector 24, which receives either a logical zero or a logical one signal from the overdrive unit 22. The selector 24 can selectively output either the driving signal 21′ or the overdrive signal 23. Thereafter, a driver IC 35 generates the control voltage VCL and the overdrive voltage VOD after processing the signals through one or a plurality of backend components 33. The backend component 33 comprises video controllers and other electronic components.
The gray scale value of driving signal 21′ in sub-pixels of each frame roughly equals the gray scale value of video signal 21. The gray scale value of overdrive signal 23 in sub-pixels of each frame can be obtained after comparing and processing the current frame with the previous frame.
The processing method of the overdrive signal 23 is as follows. When an image is displayed, each sub-pixel is either in a “dynamic state” or in a “static state” between each frame. The “dynamic state” means that a sub-pixel displays different gray scale values in the current frame and the previous frame; and the “static state” means a sub-pixel remains in the same gray scale value from previous frame.
So, the liquid crystal driving system 20 with an overdrive function relies on the video signal 21 for determining each pixel in either the “dynamic state” or the “static state” between frames. In the “static state”, because the gray scale value remains the same, the overdrive function does not have to work. The driving signal 21′ is directly generated according to the video signal 21 through the overdrive unit 22. In the “dynamic state”, the overdrive unit 22 relies on the change of the gray scale values between frames for determining the value of the overdrive signal 23.
For example, the value of the overdrive signal 23 typically is selected from one of the 256 gray scale colors of the video signal 21. Therefore, the value of the overdrive signal 23 is never out of the bound from the gray scale values of the video signal 21. For condition that both have 256 gray scale values in common, the gray scale value of the overdrive signal 23 is also selected from integers 0 to 255.
To answer the question that which gray scale value of the video signal 21 will be selected to be the value of the overdrive signal 23. The FIG. 4 and the following will explain.
FIG. 4 shows an embodiment of the overdrive signal look-up table. The value of the overdrive signal 23 in the liquid crystal driving system 20 is determined by this table.
FIG. 4 shows that each sub-pixel can display up to 256 different gray scale colors, and the gray scale 0 means white color, and the gray scale 255 means black color (Or in reverse). The initial level in FIG. 4 represents the gray scale value of the sub-pixel in previous frame, and the final level in FIG. 4 represents the gray scale value of the sub-pixel in current frame. The current frame is displayed immediately after the previous frame.
For example, from the video signal 21, if the gray scale value of the sub-pixel is 48 in the first frame, and then when it refreshes to the second frame, the sub-pixel gray scale value is 80, the value of the overdrive signal 23 can be determined as the gray scale value 192 by referring to the overdrive signal look-up table.
Referring to following table, the table shows the process from the first frame to the forth frame. The gray scale values of the overdrive signal 23 and the driving signal 21′ are shown for the liquid crystal driving system 20.
Video signalOverdrive signalDriving signal212321′1stGray scale—Gray scaleFame48482ndGray scaleGray scale—Frame801923rdGray scale—Gray scaleFrame80804thGray scaleGray scale—Frame224255
From the table above, it is not necessary for each sub-pixel to apply the overdrive function during refreshing frames. For example, the sub-pixel remains in its gray scale 80 from the second frame to the third frame. This is also known as the “static state”. It is not necessary to apply the overdrive signal 23 and the overdrive control voltage VOD. It only needs to provide the control voltage VCL matching the voltage required for the gray scale 80 for keeping the same spinning angle and maintaining brightness of liquid crystal molecules in the particular sub-pixel.
When the fist frame refreshes to the second frame and the third frame refreshes to the forth frame, the overdrive function is applied. This is known as the “dynamic state”. For example, when the first frame refreshes to the second frame, the system provides an overdrive control voltage VOD (matching the voltage for the gray scale 192) to the particular sub-pixel. And liquid crystal molecules can reach the required spinning angle within the desired time.
However, the known liquid crystal driving system 20 is unable to provide the overdrive function when the gray scale is either in the highest gray scale value or in the lowest gray scale value (white screen or black screen).
Because the gray scale values of the overdrive signal 23 are equal to the gray scale values of the video signal 21. When displaying the highest gray scale (E.g. gray scale 255), it is unable to provide a higher gray scale signal to be the value of the overdrive signal 23. For the same reason, when displaying the lowest gray scale (E.g. gray scale 0), it is also unable to provide a lower gray scale signal for applying the overdrive action.
Referring to FIG. 3 and FIG. 4. When under the “dynamic state”, the video signal 21 requests to display the gray scale 255 in a sub-pixel. The liquid crystal driving system 20 can only get the same highest gray scale reading 255 from the overdrive signal 23. This reading is not higher than the request of the video signal 21. Therefore, such an overdrive function is unable to accelerate liquid crystal molecules spinning to the desired angle.
Thus, how to improve the problem as discussed above and to provide a more refined overdrive function in a liquid crystal driving system is the primary goal of this invention.